if you just plan on 50 mining , don't bother getting it at all. It ain't even going to rise the price by a dollar.
Fletching is redundant too, too easy to obtain , and no one really respect the fletching cape now.
Heblore, if you go the fast way will cost you about 600M to 99 heblore...so ..yeah, but if you do the slow way ( cleaning herbs ) it will take you a month of two... so it's not worth it, really.
Agility is amazing and i think it is respected on a lv.3, get it. It will give value to your account.
If i were the buyer, i would prefer 99 fishing > 99 woodcutting, just my opinion.
Runecrafting is also pretty unnecessary unless your getting it real high.
With 99 agi and 99 fishing, you can easily get a pricing of like $40+.
Also , for every mil you have you can get maybe 0.80 - $1.20